We thank M. Przeworski and J. Pritchard for comments on the manuscript; P. Andolfatto, N. Berghi, B. Dumont, G. Coop, C. Ghalambor, T. Juenger, J. Kreiner, M. Nachman, M. Norborg, J. Pritchard, M. Schumer, J. Spence and M. Przeworski for helpful discussions; and US National Institutes of Health grant GM115889 and Columbia Research Stabilization Funds to GS for funding support.